1. Field of the Invention
The present invention relates to the characteristic and structure of a face of a wood club head.
2. Description of the Related Art
In order to enhance the carry distance of a hit ball, recently, there have been made various attempts to give large repulsion force from a face to a golf ball when the ball is hit by the face.
This repulsion force is based on the deformation of the face produced at impact. Generally, it is considered that the repulsion force can be increased by increasing the area of the face and by thinning the thickness of the face.
However, when such measures are enhanced, there arises a problem that the face is deformed so excessively that, for example, when a muscular golfer gives a hard impact to a ball, the durability of the face is lost. Therefore, there has been a proposal in which a rear face is disposed behind a face (that is, a front face which is to be a hitting surface) so as to form a double-face structure. The rear face is made to be substantially as large as the front face so that the load at impact on the front face is shared between both the front and rear faces.
In reality, however, it cannot be the that any face having such a double-face structure suggested hitherto brings out its own repulsion force satisfactorily.
For example, in a face having a double-face structure, as disclosed in JP-A-11-192329, a front face is too thick and it is neglected to consider the face area which can be represented by the height of the face. As a result, the increase of the carry distance expected on the basis of the repulsion force is not brought out satisfactorily in comparison with background-art products.
On the other hand, recently, United States Golf Association (USGA) established rules on the spring effect of a head face, and there arose a situation that xe2x80x9can officially recognized club is only a club having a restitution coefficient (e) which is lower than 0.83 when the entrance speed (V) of a golf ball is 48.768 m/sxe2x80x9d. In consideration of this situation as well as the durability of a face, there is a limit in the attempt that the thickness of a face having a double-face structure is made extremely thin in order to obtain high repulsion force.
In consideration of such actual circumstances, it is an object of the present invention to provide a wood club head which meets the golf rules while repulsion force of a face is brought out satisfactorily, and the durability of the face is ensured.
In order to attain the foregoing object, according to an aspect of the present invention, there is provided a wood club head having a hollow metal shell structure in which a face is constituted by a front face which is to be a hitting surface and a rear face which is disposed behind said front face through an air gap of predetermined width; wherein when a golf ball is made to collide on said front face at an entrance speed V (m/s), a restitution coefficient of said front face is in a range of from 0.84 to 0.88 in a case where said entrance speed V is in a range of from 35 to 40 (m/s), while said restitution coefficient of said front face is not lower than 0.80 but lower than 0.83 in a case where said entrance speed V is in a range of from 48 to 53 (m/s).
In order to make the characteristic of repulsion force of a face clear, the inventor of the present invention investigated, by experiments, the relationship between the restitution coefficient of a face at impact and the entrance speed (which is a speed of a golf ball colliding on a face of a head standing still according to the USGA rules, which will be described later, the speed having a value corresponding to the head speed conversely when the ball standing still is hit by the club head).
As shown in FIG. 2, generally, the restitution coefficient (e) shows a tendency to decrease with the increase of the aforementioned entrance speed (V), and has a characteristic of depending on the thickness of the face which is to be a hitting surface. That is, the restitution coefficient of the face becomes higher as the face is thinner. It is considered that this is because deformation bending of each face at impact varies in accordance with its own thickness so that its own repulsion force changes. In order to obtain large repulsion force, it will go well if the deformation is increased within the region of elastic deformation.
In addition, because the repulsion force is a spring effect, proper repulsion force cannot be obtained if deformation exceeds the yield point of the material of the face or belongs to plastic deformation.
According to the present invention, such characteristics are used in combination with one another. For average gofers, the thickness of a front face which is to be a hitting surface is set to be so thin that the restitution coefficient (e) takes a high value in a range of from 0.84 to 0.88 in the case of a general head speed (that is, the aforementioned entrance speed) of not higher than 45 m/s, particularly in a range of from 35 m/s to 40 m/s. In addition, when the head speed (V) is higher than 40 m/s and particularly lower than 48.768 m/s which is a requirement of the aforementioned golf rule, a rear face which is disposed behind the front face so as to be close thereto comes in contact with the deformed portion of the front face so as to restrain the backward deformation of the front face and reduce the value of the restitution coefficient suddenly. Thus, the restitution coefficient at the head speed (that is, the entrance speed) (V) of 48.768 m/s is set to be lower than 0.83 to meet the aforementioned golf rule. On the other hand, for highly skilled golfers, the restitution coefficient is set to be not lower than 0.80 and lower than 0.83 when the head speed is not lower than 48 m/s, particularly in a range of from 48 m/s to 53 m/s.
According to the present invention, the characteristic of the face is set to be suitable in accordance with the head speed of any player. Accordingly, it is possible to expect the increase of the carry distance on the basis of a high restitution coefficient particularly for an ordinary-skilled player whose head speed is not higher than 45 m/s and who has not been satisfied with his/her own carry distance for a long time.
In addition, the restitution coefficient at the head speed of 48.768 m/s is lower than 0.83 according to the aforementioned golf rule, so that the club can be made to meet the aforementioned USGA rules.
Further, when the head speed is not lower than 48 m/s, the thickness of the front face and the distance of an air gap provided behind the front face are selected to take proper values so that the restitution coefficient can be set to be not lower than 0.80 and lower than 0.83, which will be described later. Thus, enough carry distance and durability are ensured.
Incidentally, if the restitution coefficient is lower than 0.84 when the head speed is in a range of from 35 m/s to 40 m/s, golfers whose head speeds are low cannot obtain satisfactory carry distances. On the contrary, if the restitution coefficient exceeds 0.88, there is a fear that the front face is broken in accordance with the material thereof.
Further, if the restitution coefficient is lower than 0.80 when the head speed is in a range of from 48 m/s to 53 m/s, it is difficult to obtain enough carry distance even if the head speed is high. On the contrary, if the restitution coefficient is not lower than 0.83, there is a fear on the contravention of the aforementioned USGA golf rules or on the durability of the face.
In addition, according to a second aspect of the present invention, preferably, the thickness of the front face is made equal to or thinner than that of the rear face, and the value of the thickness is made not thinner than 1.3 mm and thinner than 2.0 mm.
Thus, it is possible to ensure necessary rigidity of the rear face which accommodates the deformation of the front face, and it is possible to produce a high restitution coefficient while the front face is made as thin as possible.
Incidentally, if the thickness of the front face is thinner than 1.3 mm, there arises a problem on the durability of the front face. On the contrary, if the front face is not thinner than 2.0 mm, enough deformation cannot be obtained at impact, so that it is not possible to bring out a sufficient restitution coefficient. As a result, the carry distance is lowered.